A children's story about a family of penguins with two fathers has again topped the list of library books the American public has objected to most frequently.

And Tango Makes Three, published in 2005 and co-written by Justin Richardson and Peter Parnell, was the most "challenged" book in public schools and libraries for the second year running, according to the American Library Association.

"The complaints are that young children will believe that homosexuality is a lifestyle that is acceptable. The people complaining, of course, don't agree with that," Judith Krug, director of the ALA's Office for Intellectual Freedom, told The Associated Press on Tuesday.

The ALA defines a "challenge" as a "formal, written complaint filed with a library or school requesting that materials be removed because of content or appropriateness."

Other books on the ALA's top 10 list include Maya Angelou's memoir "I Know Why the Caged Bird Sings, in which the author writes of being raped as a young girl; Mark Twain's The Adventures of Huckleberry Finn, long attacked for alleged racism; and Philip Pullman's The Golden Compass, an anti-religious work in which a former nun says: "The Christian religion is a very powerful and convincing mistake."

Pullman's novel, released in 1996, received new attention last year because of the film version starring Nicole Kidman.

Overall, the number of reported library challenges dropped from 546 in 2006 to 420 last year, well below the mid-1990s, when complaints topped 750. For every challenge listed, about four to five go unreported, the library association estimates.

"The atmosphere is a little better than it used to be," Krug says. "I think some of the pressure has been taken off of books by the internet, because so much is happening."

According to the ALA, at least 65 challenges last year led to one book being pulled.

In Louisville, Kentucky, a high school principal told 150 English students to stop reading Beloved, Toni Morrison's Pulitzer Prize-winning novel about an ex-slave who has murdered her baby daughter. At least two parents had complained that the novel includes depictions of violence, racism and sex.

In Burlingame, California, Mark Mathabane's Kaffir Boy, a memoir about growing up poor and black in apartheid-era South Africa, was banned from an intermediate school after a parent complained about a two-paragraph scene in which men pay boys for sex.